The present invention relates to an image forming apparatus having an image carrier in the form of a photoconductive element and forming an image on the element by an electrophotographic procedure. More particularly, the present invention relates to an image density control device installed in such an apparatus for controlling the density of the image to be formed on the photoconductive element.
Generally, an electrophotographic copier, facsimile apparatus, laser printer or similar image forming apparatus has an operation board which includes an exclusive portion accessible for adjusting an image density to the user's taste. In practice, however, the image density selected on the operation board sequentially changes due to the aging of a photoconductive element and a lamp, the shortage of toner, the fluctuation of bias voltage for development, etc. In the light of this, it has been customary to form an electrostatic latent image of a reference density pattern having a reference density on a photoconductive element outside of a document loading area, develop the latent image by a toner, sense the resulting amount of toner deposition on the latent image, and then correct the image density on the basis of the sensed amount of toner deposition. With this kind of image density control, it is possible to provide any desired image density by forming the toner density representative of the reference density pattern by an ordinary image forming procedure and by suitably selecting the reference density of the reference pattern. Specifically, the density is corrected by adjusting the output of a charger, lamp regulator or bias voltage for development or, alternatively, by on-off controlling a toner supply unit. More specifically, a toner image of a reference density pattern is formed on a photoconductive element while the amount of toner deposition thereon is sensed by a single optical sensor, so that various process units of the image forming apparatus are controlled in response to the sensed amount. An image density control device adopting this kind of principle is extensively used because the construction is simple and because the use of a single sensor saves cost. This type of prior art image density control device is disclosed in Japanese Patent Laid-Open Publication No. 57-76564, for example.
The optical sensor for the above application is usually made up of a light emitting element for illuminating the toner image formed on the photoconductive element, and a light-sensitive element to which a reflection from the drum is incident. The light emitting and light-sensitive elements are positioned such that their optical axes intersect each other at a certain angle on the surface of the photoconductive element, allowing light issuing from the light emitting element to be reflected by the toner image on the photoconductive element to reach the light-sensitive element. In this manner, the amount of toner deposition has heretofore been sensed by a single optical sensor which is constituted by a light emitting and a light-sensitive element.
The single optical sensor is usually fixed in place with its light emitting and light-sensitive elements individually held in predetermined positions, i.e., the optical axes of the two coactive elements are not changeable in direction and angle to each other. Such an optical sensor often fails to produce a sufficient output in association with the amount of toner deposition, i.e., the density of the toner image of a reference density pattern. For example, when the optical axes of the sensor are fixed at a given angle to each other, it may occur that the sensor successfully produces an output faithfully representative of the amount of toner of a toner image of a reference density pattern having a relatively high density, but it fails to do so when it comes to a toner image of a reference density pattern having a relatively low density. Experiments showed that a relatively large angle between the optical axes allows a sufficient output to be produced with a reference pattern having a relatively high density, while a relatively small angle allows a sufficient output to be produced with a reference pattern having a relatively low density. However, sufficient outputs were not achieved in the contrary situations. More specifically, the prior art optical sensors each having a light emitting and a light-sensitive element whose optical axes are fixed in the angle to each other were not capable of sensing all the amounts of toner densities associated with a plurality of reference density patterns each having a different reference density with accuracy. Thus, an optical sensor capable of sensing a broad range of toner densities, i.e., amounts of toner deposition with accuracy has been keenly demanded.